1. Field of the Invention
The invention relates in general to transport refrigeration systems of the type which hold a set point temperature by way of heating and cooling cycles, and more specifically to such systems which utilize hot compressor discharge gas for heating.
2. Description of the Prior Art
In the transportation of perishable products, it is recognized that it is necessary to provide refrigeration for the cargo space. It is also well known that it is necessary to periodically provide heat to remove the accumulation of frost or ice from the refrigeration system evaporator. Also, when transporting perishable products through areas having a cold climate, it is necessary to provide heat to the cargo space to prevent excessive cooling or freezing of the perishable products.
A well known method for providing heat for defrost and heating cycles is to divert hot compressor discharge gas from the normal refrigeration circuit, directly to the evaporator to achieve the desired heating. It has been recognized that when such a switch from a cooling cycle to a heating cycle is made, a substantial amount of the refrigerant in the system is trapped in inactive parts of the system and accordingly not available for providing heat.
U.S. Pat. No. 3,219,102 "Method and Apparatus for Deriving Heat From Refrigerant Evaporator" teaches such a heating and defrost system wherein a quantity of the hot compressed gas from the compressor is delivered to the receiver, to thereby pressurize the receiver and thus force liquid refrigerant from the receiver into the remainder of the refrigeration circuit.
Transport refrigeration systems of this type are commonly equipped with a number of safety devices. These safety devices are designed to protect system components from damage caused by unsafe operating conditions. Usually such devices are designed to sense a system parameter and compare the sensed parameter to a predetermined value of the parameter, and, to shut down the unit when such predetermined values are exceeded.
Under adverse operating conditions a transport refrigeration system of the type described in the above identified U.S. Pat. No. 3,219,102 in the hot gas heating mode may experience unacceptably high compressor discharge temperatures. With the use of higher pressure refrigerants such as R-22, which typically run at higher compressor discharge temperatures, such higher temperatures could result in compressor damage or frequent shutdown, both unacceptable.